The present disclosure relates to the subject matter contained in Japanese Patent Application No. 2001-287437 filed on Sep. 20, 2001, which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine and a printer, and more particularly to the improvement over the image forming apparatus for transferring an image formed on a photoreceptor onto a recording medium through an intermediate member.
2. Description of the Related Art
An image forming apparatus according to the related art, which adopts the intermediate transfer type, has been known in which an intermediate transfer belt as an intermediate transfer member is disposed to face a photosensitive drum as a photoreceptor. Toner images containing color components are primarily transferred onto the intermediate transfer belt in successive order. The transferred toner images are transferred, at a time, from the intermediate transfer belt onto a recording medium, e.g., a recording sheet of paper.
In this type of the image forming apparatus, the intermediate transfer belt is usually constructed as a belt unit in which the intermediate transfer belt is wound on a plurality of rolls and circulatively moves in a predetermined travelling direction.
In the belt unit, the intermediate transfer belt moves not straightly while being displacing in an axial direction of tension rolls that is, so-called meandering occurs. The meandering of the intermediate transfer belt is caused by variations in the parallelism among rotary shafts of the tension rolls, variations in the outside diameters of the tension rolls, and a non-uniformity of tensions caused by a variation of the periphery length of the intermediate transfer belt per se.
When the meandering occurs, transfer positions of the toner images, which are transferred onto the intermediate transfer belt in successive order, are displaced from the correct ones. As a result, there is a possibility that such image defects as color mis-registration and hue variation occur in the color image formed on the recording medium, e.g., a recording sheet.
Some measures have been taken for the meandering of the belt. One of the measures is that the end of the belt is abutted against the edge guide to position the belt in place. Another measure is that ribs are formed at the ends of the rear side of the belt to control the meandering motion of the belt. Yet another measure is that belt end positions are detected by position sensors, and the parallelism of a desired tension roll is variably adjusted according to the sensing results.
In those measures, the edge guides and ribs for abutting are provided on both ends of the belt. The position sensors for detecting both end positions of the belt are also provided at both ends of the belt. However, the belt will finally displace by variations of the parallelism and diameters of the tension rolls.
In such circumstances, the reinforcing structure for avoiding the damage of the belt by stress concentration on the butting edge guide, and bonding of the ribs are high in cost. Further, it is disadvantageous in the light of space saving that the belt width is larger than the image forming width.
To cope with this, a technique, which employs a technique for shifting the belt to one side, simplifies the structure, and stabilizes the meandering correcting performance, already has been proposed.
There has been a known technique for shifting the belt to one side in which the diameter of one tension roll is gradually varied in the axial direction to shift the belt to one side (e.g., JP-A-Hei.6-278894) and has been another known technique to incline the parallelism of a desired tension roll.
After or when it is mounted on the apparatus body, due to distortion of the belt unit, although the parallelism of each tension roll of the belt unit is kept, the tension rolls may be slanted in the same direction by the distortion.
Under the circumstances, when the technique for shifting the belt to one side (the technique for gradually varying the diameter of the tension roll or the technique for inclining the parallelism of one tension roll) is employed, depending on the distortion direction of the belt unit, it is feared that the belt is directed to a side which is opposite to the side as intended.
To set the gradual variation of the diameter of the tension roll and the inclination of the parallelism of the tension rolls in consideration with the twist of the belt unit, the variation and inclination must be set large. If the variation and inclination are set too large, this becomes cause of tension wrinkle so that it is feared to have disadvantage in performance of transfer.
Incidentally, such problems does not only arise in the technique for shifting the belt to one side, but also similarly arise in the technique for restricting the meandering of the belt at both side because in many cases, the belt is shifted to any of sides.
Accordingly, the present invention has been made for solving the above problems and provides the following technique: even when a twist occurs in the belt in a state that the parallelism of a tension roll of the belt is kept, the belt may be shifted to one side in a desired direction without causing tension wrinkles and the like in the belt, and hence, the meandering control of the belt is alleviated in load. With this technique, the invention provides an image forming apparatus which can reduce the color mis-registration and can perform good image formation.
According to the invention, there is provided an image forming apparatus having a photoreceptor, an intermediate transfer member, and a plurality of tension rolls, in which one of the photoreceptor and the intermediate transfer member is formed like a drum, in which the other of the photoreceptor and the intermediate transfer member is formed like a belt, which is wound on the plurality of tension rolls, and in which the following relationship is satisfied:             ∑              n        =        3            N        ⁢          a      n         less than   x
an is the contact lengths between the other and the n tension rolls, respectively and x is contact length between the photoreceptor and the intermediate transfer member.
According to the present invention, as shown in FIG. 1A, there is provided an image forming apparatus for transferring an image formed on a photoreceptor 1 onto a recording medium 3 through an intermediate transfer member 2, in which one of the photoreceptor 1 and the intermediate transfer member 2 is shaped like a drum, while the other is shaped like a belt, which is put on a plurality of tension rolls 4, and the following relation is satisfied:
a+b+c+ . . .  less than x 
where a, b, c, . . . are contact lengths between the other of the intermediate transfer member 2 and photoreceptor 1, which are formed in a belt like shape, and each of tension rolls 4 and x is contact length between the intermediate transfer member 2 and the photoreceptor 1.
It should be understood that the present invention thus technically implemented is applied to a variety of image forming apparatus. It is preferable that the invention is applied to the color image forming apparatus since remarkable technical effects are produced in such applications. However, it should be understood that the invention may be applied to the monochromatic image forming apparatus, as a matter of course.
In relation to a mode of the photoreceptor 1 and the intermediate transfer member 2, one of them is formed in a drum shape, and the other is formed in a belt shape. A belt-like intermediate transfer member 2 is used for a drum-like photoreceptor 1 (the combination of them is illustrated in FIG. 1). A drum-like intermediate transfer member 2 is used for a belt-like photoreceptor 1.
Further, as a measure for the meandering of a belt like member (hereinafter, referred to simply as a belt), in view of simplification of the structure of the apparatus, it is preferable that the intermediate transfer member 2 or photoreceptor 1, which is formed in the belt like shape, adopts a technique for shifting the belt to one side. It should be understood that the invention is not limited to the technique for shifting the belt to one side. The invention also includes the technique for restricting the meandering at both sides of the belt. This is because in the technique for restricting the meandering at the both ends of the belt, in many cases, the belt also is intended to shift to any of the sides due to error in a part/assemble and therefore there is enough worth to use the invention.
In the invention, to specify a contact region (contact length x) between the photoreceptor 1 and the intermediate transfer member 2, it is taken into account to a balance between an inside of the belt and an outside of the belt and an axis of the drum like shaped member (hereinafter referred to as a drum) is inclined, whereby the contact length in the inside of the belt (the contact length in relation to the tension roll 4) is made larger than the contact length in the outside of the belt (the contact length in relation to the drum) so that a shift direction of the belt can be restricted uniformly.
To be more specific, for example, in an example shown in FIG. 1, when twist occurs in a state that the parallelism among the tension rolls 4 of the belt-like intermediate transfer member 2 (belt) is kept, in a case where only the tension rolls 4 being in contact with the inner surface of the belt 2 are provided, all the tension rolls 4 are twisted in the same direction to apply axial directional forces FR in the same direction to the belt 2 as shown in FIG. 1B. However, when the drum-like photoreceptor (drum) 1 being in contact with the outer periphery of the belt 2 is twisted in the same direction as that of the tension rolls 4, an axial directional force FD in an opposite direction to that of the tension rolls 4 being in contact with the inner surface of the belt 2 is applied to the belt 2.
When the belt unit is thus twisted, in a case where only the tension rolls 4 being in contact wit the inner surface of the belt 2 is provided, it is feared that a force (the force FR in FIG. 1B) to shift the belt to one side in a opposite direction to that as intended.
On the other hand, the drum 1 being in contact with the outer periphery of the belt 2 produces a force (the force FD in FIG. 1B) in the opposite direction to that of the tension rolls 4 being in contact with the inner surface of the belt 2.
A force to shift the belt 2 to one side is canceled in a state that a+b+c+ . . . =x, where a, b, c, . . . are each contact length between each of tension rolls 4 and the belt 2 and x is a contact length between a surface of the drum 1 and of the belt 2.
In order to shift the belt 2 to the intended direction, it is necessary to make a+b+c+ . . .  less than x or a+b+c+ . . .  greater than x. However, the condition xe2x80x9ca+b+c+ . . .  greater than xxe2x80x9d implies that the tension rolls 4 on the inner surface of the belt are all slanted to the intended direction. It is very difficult to satisfy this condition. Accordingly, the invention employs the condition xe2x80x9ca+b+c+ . . .  less than xxe2x80x9d and adopts a technique for shifting the belt 2 to one side by slanting the drum 1 being in contact with the outer periphery of the belt 2.
The photoreceptor 1 and the intermediate transfer member 2 maybe driven by separate drive systems, respectively. Since the photoreceptor 1 and the intermediate transfer member 2 are brought into contact with each other over a relatively large area, one of the photoreceptor 1 and the intermediate transfer member 2 may be used as a drive source and the other may be rotated following the drive source.
In this mode, one of the drive mechanisms can be omitted and the size and cost of the image forming apparatus are reduced correspondingly. Further, an effect of shifting the belt (e.g., the belt-like intermediate transfer member 2) to one side is more effectively produced.
The other of intermediate transfer member 2 and the photoreceptor 1, which is belt like, may be made of an appropriate material such as resin, rubber, or the like. Preferably, the other of the intermediate transfer member 2 and the photoreceptor 1, which is belt like, includes at least an elastic layer.
With this structure, the friction coefficient of the belt (e.g., belt-like intermediate transfer member 2) is increased, whereby the belt can be effectively shifted to one side.
To secure a satisfactory cleanness property for soil on the belt surface, preferably, the other of the intermediate transfer member 2 and the photoreceptor 1, which is belt like, has a multi-layer structure including a release layer formed at least on the surface of the elastic layer.
Furthermore, preferably, the elastic layer has the Young""s modulus in a range of from 15 Mpa to 80 MPa.
An appropriate number of tension rolls 4 for the belt may be used. To lessen or stabilize the influence by the meandering of the belt, it is preferable to put the other of the photoreceptor 1 and intermediate transfer member 2, which is belt like, on at least four or more number of tension rolls 4.
In this mode, the four tension rolls 4 are used as a pair of tension rolls for positioning with respect to the drum-like counter member, or used as backup rolls for the secondary transfer and the cleaning.
In view of effectively performing the operation of shifting the belt to one side, it is preferable that the one of the drum-like photoreceptor 1 and intermediate transfer member 2, which is drum like, are disposed at a location where a distance between the tension rolls 4 for the one of the intermediate transfer member 2 and the photoreceptor 1, which is belt like, is the longest among distances between the tension rolls.
The feature of the xe2x80x9clocation where the distance between the tension rolls 4 is the longestxe2x80x9d lessens the influence of the operation of shifting the belt to one side by the adjacent tension rolls 4 on the operation of shifting the belt (e.g., intermediate transfer member 2) to one side by tilting the drum (e.g., drum-like photoreceptor 1).
The present invention is based on the construction including the photoreceptor 1 and the intermediate transfer member 2. The invention may also be applied to a construction including the photoreceptor and a recording medium transporting member for transporting a recording medium so long as one of the photoreceptor and the recording medium transporting member is shaped like a drum, and the other is shaped like a belt.